Cathodic sputtering is widely used for the deposition of thin layers of material onto desired substrates. Basically, this process requires a gas ion bombardment of the target having a face formed of a desired material that is to be deposited as a thin film or layer on a substrate. Ion bombardment of the target not only causes atoms or molecules of the target material to be sputtered, but imparts considerable thermal energy to the target. This heat is dissipated by use of a cooling fluid typically circulated beneath or around a backing plate that is positioned in heat exchange relation with the target.
The target forms a part of a cathode assembly which together with an anode is placed in an evacuated chamber that contains an inert gas, preferably argon. A high voltage electrical filed is applied across the cathode and anode. The inert gas is ionized by collision with the electrons ejected from the cathode. Positively charged gas ions are attracted to the cathode and, upon impingement with the target surface, dislodge the target material. The dislodged target materials transverse the evacuated enclosure and deposit as a thin film on the desired substrate that is normally located proximate the anode.
The targets may comprise metals such as, for example, aluminum, copper, tantalum, titanium, or tungsten. The materials from the target itself may be sputter coated onto the substrate or, in some cases, compounds formed from the target material and a process gas may be formed on the desired substrate. Examples of this “reactive” sputtering include tantalum nitride, titanium nitride, and tungsten nitride compounds that are coated onto the desired substrate during the sputtering process. Typically, the sputtering chamber comprises a housing which encloses a process zone into which the process gas, such as N2, is admitted during the reactive sputtering process.
Fabrication methods by which targets are manufactured typically create a damaged surface layer of the target that produces undesirable or inconsistent sputtering properties. For example, machining of the target surface requires shearing force exertion on the target surface that can plastically deform and create other defects in the surface grains. These defects results in variable and non-uniform sputtering properties across the target surface.
Normally, a target “burn in” step is used to remove the undesirable damaged surface layer of the target. This burn in step is performed in the sputter chamber with the target exposed to the excited plasma gas to result in sputtering off of the undesirable surface layer. Obviously, this burn in process is costly in terms of energy expenditure and sputtering chamber down time.
A variety of attempts have been made to remove the undesirable target surface deformation later such as by grinding, electropolishing, chemical etching and chemical/mechanical polishing (CMP). Grinding processes often result in the deposition of embedded media in the target surface with electropolishing and chemical etching potentially resulting in target surface H2 retention. CMP processes produce sludge that can contaminate other surfaces.